This invention relates to apparatus for applying power to at least two heater filaments of two glow plugs each projecting into a combustion chamber in a diesel engine, or to a heater filament of one glow plug and a ballast load having substantially the same maximum rated voltage as the filament. The glow plugs are heated by applying a source of power to the filament contained therein. The heated glow plug facilitates diesel engine starting by raising the temperature of air in the combustion chamber from ambient temperature to an operating temperature sufficiently high to start the engine. Therefore, an operator of the engine must wait a relatively substantial period of time before the glow plugs in the engine have been sufficiently heated to facilitate diesel engine starting.
The voltage source for the filaments can be, for example, a conventional vehicle battery which is also used to energize a starter for the engine. A decreasing battery voltage lengthens the period of time the operator must wait before the glow plugs have been sufficiently heated. One method of controlling filament temperature is to employ direct temperature feedback from the filaments. However, such a method requires more complex and expensive electronic components as well as a direct connection to the filaments during engine operation. Another problem involves de-energizing the filaments within some time period after the engine has started to prevent overheating and subsequent burnout.